Filamentation also refers to the self-focusing of a high power laser
pulse. At high powers, the nonlinear part of the index of refraction
becomes important and causes a higher index of refraction in the center
of the laser beam, where the laser is brighter than at the edges,
causing a feedback that focuses the laser even more. The tighter focused
laser has a higher peak brightness (irradiance) that forms a plasma.
The plasma has an index of refraction lower than one, and causes a
defocusing of the laser beam. The interplay of the focusing index of
refraction, and the defocusing plasma makes the formation of a long
filament of plasma that can be micrometers to kilometers in length.[34] One interesting aspect of the filamentation generated plasma is the
relatively low ion density due to defocusing effects of the ionized
electrons.